1. Field of the Invention
The present invention relates to an optical rotary encoder, a control method therefor, and a coordinate input device using the optical rotary encoder.
2. Description of the Related Art
In recent years, GUIs (Graphical User Interfaces) have been employed so as to enhance the operability of PCs (Personal Computers). Coordinate input devices, such as mice and track balls, have been used to as to point at icons indicated on the screens of display units.
In such a coordinate input device, an optical rotary encoder is frequently used so as to resolve each of the amount and direction of rotation of a ball contained therein into orthogonal components respectively corresponding to two directions, that is, the X-direction and the Y-direction, and so as to measure the orthogonal components.
Generally, the optical rotary encoder has light emitting devices or elements, light receiving devices for receiving light emitted from the light emitting devices, and a slitted disk. The disk is disposed between the light emitting devices and the light receiving devices. When the disk rotates, the slits of the disk transmit the light emitted from the light emitting devices. Conversely, the remaining non-transparent parts of the disk interrupt the light emitted from the light emitting devices. The light receiving devices receive the light transmitted through the slits. The received light is converted into electric pulse signals corresponding to the speed and direction of rotation of the disk.
The coordinate input device using the optical rotary encoder detects the amount and direction of movement of the operated ball as the amount and direction of rotation of the disk of the optical rotary encoder.
Generally, a set of light emitting diodes (LEDs) and phototransistors is used as a set of the light emitting devices and the light receiving devices of the optical rotary encoder. The phototransistor is a device containing a photodiode and a transistor, and has a light or optical amplification function of amplifying a signal, which is obtained by a photoelectric conversion in the photodiode, by the transistor.
LEDs and transistors are analog devices, so that there is an individual difference in characteristic among the LEDs or the transistors. That is, there is an individual difference in, for example, luminous intensity among LEDs. Further, there is an individual difference in, for instance, light receiving sensitivity among phototransistors. When such an individual difference is large, the extraction of information on the amount and direction of rotation of the disk of the optical rotary encoder is difficult owing to a subtle change in temperature and to deterioration with time of devices. Especially, in the case of using an optical rotary encoder in the coordinate input device, a mouse cursor cannot move normally.
Therefore, generally, when an optical rotary encoder using LEDs and phototransistors is designed, a desired magnitude of the optical coupling between the LEDs and the phototransistors is obtained by adjusting the constants of the devices of various kinds of processing circuits and adjusting the distance between the LEDs and the phototransistors so as to eliminate the individual difference among the devices. Consequently, stable outputs of the optical rotary encoder are secured.
Moreover, generally, LEDs and phototransistors are classified into what are called xe2x80x9cranksxe2x80x9d based on the aforementioned individual differences. For example, LEDs are classified into ranks according to relative levels (or luminosity rank) of a luminous amount. On the other hand, phototransistors are classified into ranks according to relative levels (or reception light intensity ranks) of a photocurrent flowing therethrough when light of the same amount is irradiated thereon. When an optical rotary encoder is designed, the ranks of such devices are sufficiently taken into consideration. An optimal combination of LEDs and phototransistors, which ensures a stable operation of the optical rotary encoder, is selected.
The aforementioned circuit adjustment and the adjustment of the distance between LEDS and phototransistors at the time of fabricating an optical rotary encoder are costly and unreliable. Furthermore, when an optical rotary encoder is designed by considering the luminous ranks and the reception light intensity ranks, the range of choices of available devices is narrow. Consequently, the fabrication costs of the optical rotary encoder are high.
Besides, in the case of using, for example, the aforementioned optical rotary encoder as the coordinate input device, it is possible to input an output of the phototransistor directly to a microcomputer serving as a coordinate processing unit in the coordinate input device. However, the outputs of the phototransistors need adjustment because of the fact that the threshold voltage of the input port of the microcomputer changes according to the specification thereof.
In recent years, devices called photo ICs have been supplied to the market as light receiving devices obtained by improving the phototransistors that have such defects. Optical rotary encoders employing photo ICs as the light receiving devices are currently on the market.
An ordinary phototransistor has two photodiodes, while a photo IC has four photodiodes and hundreds of transistors. In a photo IC, two pairs of photodiodes are used and adapted so that each pair of photodiodes generates two signals, whose phases are inverted, from received light, that a differential signal representing the difference between the two signals is then obtained, and that an electric pulse signal is outputted by shaping the waveform of the differential signal into a rectangular waveform.
Therefore, photo ICs have an advantage in resistance to change in temperature and humidity and to variation in amount of a luminosity (that is, to deterioration with time of an amount of a luminosity) over ordinary phototransistors having only the optical amplification function.
Moreover, because output signals of a photo IC are electric rectangular pulse signals, namely, binary signals each having a high level or a low level, photo ICs have another advantage in that output signals of a photo IC can be inputted directly to a microcomputer of a coordinate input device when the photo IC is used in, for instance, an optical rotary encoder of the coordinate input device.
Photo ICs are integrated circuits, and have another advantage in that each photo IC can have various additional functions in addition to a light receiving function. For example, in the case that LEDs serving as light emitting devices are intermittently driven, and that light emitted by the LEDs is received by a photo IC, the photo IC can have a data holding function by which data obtained by the photo IC at the time of receiving light is stored in an internal memory even when the LEDs are turned off.
Thus, a device using photo ICs in an optical rotary encoder has many advantages over a device using phototransistors in an optical rotary encoder. However, the former device has many drawbacks.
First, the former device using the photo ICs has a drawback in that the photo IC is an integrated circuit and always consumes electric current even in the case of receiving no light, different from the latter device in which ordinary phototransistors fundamentally output no electric current when receiving no light.
Although the photo IC always consumes electric current, the value of the electric current varies from hundreds xcexcm to several mA and thus the photo IC does not impede an operation of a coordinate input device having an interface, such as a PS/2 (Personal System/2) interface, in the case that the photo IC is used in such a coordinate input device. However, in the case of the device using a USB (Universal Serial Bus) interface that has been becoming widespread in recent years, generally, electric current consumption in a suspended mode of a coordinate input device is prescribed by standards. The suspended mode is also referred to as a standby mode. In such a mode, for example, performance of a normally available function is temporarily paused, or such a function is tentatively disabled. Thus, the power consumption of the device is reduced in comparison with that in an ordinary operation mode of the device. Consequently, power saving is achieved. Incidentally, although the term xe2x80x9ca sleep modexe2x80x9d is used in a similar sense, it is understood by those skilled in the art that the power consumption in the suspended mode is much lower than the power consumption in the sleep mode.
The standards prescribe that electric current consumed in the suspended mode of a peripheral device, such as a coordinate input device using a USB interface, should be less than 500 xcexcA. However, the electric current consumed per photo IC is 200 xcexcA at most. Further, an electric current of about 250 xcexcA is consumed only by communication lines. Thus, the total consumption of electric current of the coordinate input device using two photo ICs is 650 xcexcA, which exceeds the value prescribed by the standards.
Although the coordinate input device using photo ICs as the light receiving elements of the optical rotary encoder thereof has many advantages over the conventional device using phototransistors, especially, a reduction in electric current consumed in the suspended mode of the device is still an outstanding problem.
Accordingly, in view of the aforementioned problem, a first object of the present invention is to provide a low power consumption optical rotary encoder employing photo ICs as light emitting devices.
In view of the foregoing problem, a second object of the present invention is to provide a coordinate input device having a low power consumption optical rotary encoder that employs photo ICs as light emitting devices.
In view of the aforementioned problem, a third object of the present invention is to provide a control method of controlling a low power consumption optical rotary encoder that employs photo ICs as light emitting devices.
To achieve the first object of the present invention, according to a first aspect of the present invention, there is provided an encoder having a light emitting device and a photo IC for receiving light emitted from the light emitting device. This encoder comprises driving unit having a first driving power output terminal, to which one of a pair of power supply terminals of the photo IC is connected, for outputting driving power to be used for intermittently driving the photo IC only for a desired time period.
To achieve the second object of the present invention, according to a second aspect of the present invention, there is provided a coordinate input device that comprises the encoder according to the first aspect of the present invention and a slitted disk in which a plurality of slits are formed along a circumference at uniform intervals. This coordinate input device generates coordinate input data, which is to be inputted to a computer, according to slitted-disk rotation information.
To achieve the third object of the present invention, according to a third aspect of the present invention, there is provided a control method of controlling an encoder that comprises a light emitting device and a photo IC for receiving light from the light emitting device. According to this control method, electric power for intermittently driving the photo IC for a desired time period is outputted from a first driving power output terminal to which one of a pair of power supply terminals of the photo IC is connected.
In the case of the optical rotary encoder employing the photo IC as the light emitting device according to the first aspect of the present invention, the power consumption of the encoder can be reduced by intermittently supplying electric power to the photo IC as driving power therefor. Further, the power consumption can be reduced still more by intermittently driving the light emitting device of the encoder.
In the case of the coordinate input device according to the second aspect of the present invention, the encoder enabled to intermittently supply electric power to the photo IC as the driving power supply therefor is used for detecting the amount and direction of movement of the coordinate input device. The power consumption of the coordinate input device can be reduced.
In the case of the control method of controlling the optical rotary encoder, which employs the photo IC as the light receiving device, according to the third aspect of the present invention, electric power can intermittently be supplied to the photo IC as a driving power supply therefor. Further, the light emitting device of the encoder can intermittently be driven. The power consumption can be reduced still more.